This invention involves a process for coating circuit boards, especially those with through-plated holes, with photopolymerizable, solvent-free, thermoplastic material.
Research Disclosure 249019, January 1985, and European Patent 00 80 665 describe processes for mixing a photopolymerizable compound by melt extrusion and coating the melt on a support. Such processes have been used industrially to manufacture photopolymerizable products, such as flexographic printing plates, by coating the photopolymerizable composition on the essentially fiat surface of the support.
However, in the manufacture of circuit boards, individual boards with a surface already populated by holes or conductor lines are coated by the user.
Such coatings of resist materials serve, on one hand, in forming the conductor lines and on the other hand, in protecting them during solder processes. The coatings are required during various process steps to protect the underlying material from attack by processing baths, such as developers, etchants, and electroplating agents.
Whereas resist material is applied imagewise by screen printing methods when requirements in conductor line precision are low, photographic methods are used when fine patterns require high resolution. In the latter method, the resist layer is first coated uniformly on the entire surface. Then the layer is exposed imagewise through a transparency and developed by washing out the exposed areas (for positive-working resist materials) or the unexposed areas (for negative-working resist materials) of the resist image.
Various coating processes are known and adapted to the properties of the photoresist materials required for different uses, as follows.
Liquid photoresists, mostly based on diazo compounds, are applied by spin coating or by dip coating the support. They can also be applied continuously by various coating processes, particularly curtain coating (as, for example, in U.S. Pat. No. 4,230,793) or by spray processes, especially electrostatic spray processes.
Electrophoretic deposition produces the resist layer with another group of liquid photoresists.
These processes are suitable for liquid photoresists having low viscosity due to the addition of solvents. Therefore, the boards must be dried after the coating step.
Another coating process is roller coating, whereby the liquid coating material is applied by rollers onto the surface to be coated. The rollers usually have surfaces structured to accept adequate coating material (PCT/WO 92/07679).
Such liquid photoresists are relatively expensive and are particularly suitable for making thin layers. However, they have the disadvantage that both sides of the circuit board cannot be coated simultaneously, a uniform coating requires great technical effort, and the unhardened resist surface is very sensitive to dust and mechanical damage. It has also been found difficult to coat populated circuit boards with solder mask, because vertical surfaces are not satisfactorily coated by the liquid resist. In particular, protecting through-plated holes is difficult. Although the liquid resist penetrates into the holes easily due to its low viscosity, it does not fill the holes completely and reliably, because it is difficult to expel air from the holes. Therefore, through-platings are not sufficiently protected and can be destroyed or attacked during etching.
Electrophoretically deposited, liquid photoresists yield an especially uniform coating. However, they require conductive substrates and therefore, cannot be used, for example, as solder masks. Coating thickness is limited to a maximum of about 15 m, which is too thin for an electroplating resist; therefore, they can be considered only as etching resists.
As with low viscosity, liquid photoresists, this method also has specific drawbacks in coating circuit boards with through-plated holes. The thin layer does not fill the holes completely; many holes have only their walls covered. To protect the holes with negative working photoresists, the resist layers on the walls must be hardened reliably by exposure. This is not definitely assured in holes with a high aspect ratio (depth/diameter).
With positive working resists, on the other hand, it is necessary to reliably prevent the holes from being exposed during the entire processing sequence. This is possible only by ensuring no exposure of defined surface area exposure and protecting such surface areas in all succeeding process steps from actinic light.
Many of the above disadvantages are eliminated by dry resist films, which consequently have become widely used. These are described, for example, in U.S. Pat. Nos. 3,469,982 and 3,547,730. They have a sandwich structure with a photopolymerizable layer between a temporary support and a transparent cover sheet. In most cases, the photopolymerizable layer is transferred onto the circuit board, is exposed imagewise through a circuit pattern transparency, and after the removal of the cover sheet, is washed out with a suitable solvent.
Dry resists are outstanding for their simple handling, for a layer thickness very accurately adjusted by the producer, and for the resist surface being well protected by a cover sheet. However, their ability to conform to structured supports is limited. A special disadvantage is that holes can be protected only by tenting. This process involves covering the holes with photoresist film. Care must be taken during exposure to assure that the resist is exposed not only over the hole but also on an larger anchoring area around the hole. This requires additional space on the circuit board and can lead to space problems with the increasingly higher packing density of the circuits. In addition, this does not always assure that large holes are protected reliably by tenting.
Therefore, the problem involved in this invention is to develop a process for coating circuit boards, especially circuit boards having holes, with photopolymerizable material, whereby holes can be protected reliably without lateral overlays, drying is not needed, the process is not complicated to perform, and it produces coatings, about 5 to 100 m thick, that conform well to possibly populated supports.